I have had issues getting MQTT working for AWS even using the Amazon Root CA1 for the ca_certs file.
Have used example aws_https_pc.py to verify connection to AWS (with same key / ca / crt files.)
When using digi example to connect using umqtt.simple, connect() never returns a valid result. Instead, returns:
File "/flash/lib/umqtt/simple.py", line 71, in connect
OSError: [errno 7005] EIO